1. Field of the Invention
The invention relates generally to subterranean well tools such as inflatable packers, bridge plugs or the like, which inflate through the introduction of fluid into an expandable elastomeric bladder and, more particularly, to a spring-loaded apparatus and method for maintaining a relatively uniform fluid pressure in the bladder when the tool is subjected to thermal variants after expansion.
2. Description of Problems
It is known among those skilled in the use of these types of inflatable devices that they are subject to changes in inflation pressure when the temperature of the inflation fluid varies from its initial inflation temperature. Typically, an increase in fluid temperature results in increased inflation pressures, and a decrease results in decreased inflation pressures. An increase in inflation pressure can make the tool susceptible to burst failure. A decrease in inflation pressure can diminish anchoring between the tool and the well bore to a point where the tool is not able to provide its intended anchoring function. In both instances, significant changes in temperature in the inflation fluid can result in compromised tool performance and possible tool failure. These failures can result in significant monetary loss and possible catastrophe.
The magnitude of temperature change needed to adversely effect the performance of an inflatable tool depends upon a number of parameters, such as, for example (1) the expansion ratio of the inflation element, (2) the relative stiffness of the steel structure of the inflation element compared with the compressibility and thermal expansion coefficient of the inflation fluid, (3) the relative stiffness of the casing and/or formation compared with the compressibility and thermal expansion coefficient of the inflation fluid, and (4) the anelastic properties of the elastomeric components in the inflation element. There are other factors of lesser significance known to those skilled in the relevant art.
Regardless of the specific values of the aforementioned parameters, conventional inflatable tools cannot tolerate positive or negative temperature changes greater than about 10-15.degree. F.(5.6-8.3.degree. C.) from the initial temperature at the end of their inflation cycle. If the temperature of the inflation fluid varies by more than this amount, the tool is subjected to excessive inflation pressures or insufficient inflation pressures, which could result in tool performance problems of the nature described above.
In addition, cycling the inflation fluid temperature within a .+-.15.degree. F. of the initial temperature upon expansion can cause stress cycling in the steel structure of the inflation element and in the bladder. There is the potential for a serious problem when the inflation element survives routine thermal cycling for a finite period of time, during which cyclic damage in the tool accumulates. In such a case, failure can occur at some time after the rig has departed from the well site. Thus, an inflatable tool can provide short term functional performance during low magnitudes of thermal cycling. However, cumulative damage phenomena can occur in steel structures and/or elastomeric components and eventually cause device failure.
A time delayed failure can be more costly and possibly more catastrophic than one which occurs within a short time after the initial setting of the tool. Replacement of the failed device would entail performing a second project about equal in size and expense to the first service operation, instead of the case of a short-lived tool which would fail before the rig is broken down and moved off the site. Operations of this type can cost in excess of one hundred thousand dollars, and as high as several millions of dollars.
There are many operations in the oil and gas industry that successfully use pressure isolation devices which routinely encounter substantial thermal excursions and substantial magnitudes of combined positive and negative thermal cycling. Typically, inflatable devices are excluded as candidates for such projects. Typical projects are listed below.
large volume stimulation projects, n PA1 selective zone treatment projects, n PA1 large volume cement squeeze projects, n PA1 production packer service in oil and/or gas wells experiencing cooling from Joules-Thompson expansion and cooling of gases, n,c PA1 production packer service in oil and/or gas wells experiencing heating from deeper produced fluids, p,c PA1 conversion of a producing well to an injection well and temporary isolation between perforation intervals, n,c PA1 huff/puff steam injection methods for producing viscous oil formations, p,c
[n=these operations typically result in a large negative thermal excursion (cooling) in the pressure isolation device.] PA0 [p=these operations typically result in a large positive thermal excursion (heating) in the pressure isolation device.] PA0 [c=these projects typically repeated multiple thermal cycling in the pressure isolation device over long periods of time.]
The first five project categories are very common in the industry. Thousands of them are performed per year. The bottom two categories are relatively infrequent with respect to world wide activities.
If conventional packers and bridge plugs are not able to provide service for a given well configuration, because they are not able to pass through restrictions and subsequently set in casing, it is common to use a rig to pull tubing and perform a costly work-over project.
The use of thru-tubing inflatable devices provides well known benefits and versatility to the oil and gas industry. Their lack of service worthiness for operations that include thermal cycling and thermal excursions exclude them from a substantial portion of the remedial service sector. An invention that would eliminate the deleterious effects of routine thermal excursions and thermal cycling, would eliminate the aforementioned problems, augment the benefits and versatility of inflatable devices and provide substantial cost savings to operators in the industry.